The invention concerns a method for the position-dependent control of a mobile element in a motor vehicle, wherein the element is movable in a drive movement by an electrically powered positioning device, according to the preamble of Claim 1.
Accordingly, the positioning device has several different operational modes, and characteristics of a drive signal of the positioning device, in particular a waviness of a drive current, are used for the determination of the position of the element.
The different operational modes of the positioning device can in particular contain the activation and deactivation of the automatic operation of the mobile element in one of its end positions and the adjustment of the position of the mobile element upon reaching an assigned position. However, a plurality of combinations of different functional features of the positioning device is also possible, which represent different operational modes of the positioning device.
As a characteristic feature of the drive signal, for example, the time dependent waviness of the drive current or the drive voltage for an electric motor with or without a commutator may be evaluated. Likewise, it is possible to evaluate the frequency dependent waviness of the drive current or the drive voltage in the frequency domain.
With commutating brush-type DC motors, the method according to the preamble is known as “ripple count.” With such electric motors, the motor load current is superimposed with current impulses (ripples). These current peaks are generated by the periodic reversion of the rotor caused by the commutator. Advantage is then taken of the fact that the path covered by the mobile element through the electric positioning device correlates with the time dependent occurrence of the current peaks of the drive current of the positioning device. That is, a defined track section on the positioning path of the mobile element can be assigned to each period between two current peaks of the drive current. If the current peaks are counted during the positioning movement of the mobile element, the positioning path can be calculated.
The “ripple-count” method is equally applicable to an electric motor without commutator. Here, the period of the AC current necessary for driving the rotor serves as a characteristic feature of the drive signal. A defined track section on the positioning path of the mobile element can be assigned to each period, for example, between two maxima of the drive current.
The “ripple-count” method works with brush commutating DC motors without problems as long as a critical load torque does not have to be exceeded during movement of the mobile element, or the electric motor is not in the starting or switching off phase. In these critical operating states, the current impulses for the determination of the positioning path are difficult to evaluate, or cannot be evaluated at all. This leads necessarily to a deviation between the position of the mobile element determined using the current impulses and its real position. This error leads, in particular when the “ripple-count” method is used for power windows of automobiles, to the following problem. The power window's crush guard may only be deactivated during the automatic closing process when the window's opening gap is only four millimeters wide. If the error in determining the position is, for example, in the range of a tenth of a millimeter, this legal requirement can no longer be reliably met.